Ian, I have never 'pre-mixed' to a stereo pair. (not even sure how to do that?) I've always just merged the entire RB track w/ DX to a stereo .wav file using the selection as noted above. I USED to be able to produce stereo files until recently.. and it's driving me nuts trying to find out what happened.

My wav file ends up sounding exactly like it sounds if I check the 'mono' box in the Mixer Window... it's as if when rendered, RB 'thinks' that mono box is checked ?